There ought to be an automatic small penalty (say 50p or £1) added to tickets bought on the train when boarding at a station that has ticket buying facilities. I see loads of (sadly usually youngsters) getting on right at the front in the hope of the guard not reaching them before they get off one stop later, and when he does they always seem to have to buy a ticket from him. A small surcharge might be enough to deter them from taking the risk.
Of course for this penalty to be applied the guard still has to get to them and try to issue a ticket , and if there is a few of them there are always going to be some who get a free ride . If they argue with the guard long enough about paying the penalty and just chuck the usual money for a single at the guard before getting off there is not much that can be done .
This is one of the things when you look at fare evasion , without having every station manned with a police prescence you will never get fare evasion down to 0% , there will always be some that get away with it . The key is making it harder to fare evade without spending disproportionate sums on measures to tackle fare evasion .
I would also question the notion that it is usually youngsters , I can think of several occasions when it has been older people doing the same . I can also think of countless occasions when I have caught pensioners with free passes trying the same in peak time when their pass is not valid . In my experience its not just youngsters , nor in my experience do youngsters make up a significant majority of those pay when challenged getting on furthest away from the guard .
There already is a surcharge as such - the NRCoT say that, if you aren't issued with a Penalty Fare and don't have your details taken for prosecution, you will still have to pay the undiscounted Anytime (Day) Single.
Of course in some cases the undiscounted anytime single is the only single fare that is available and the passenger might only want a single and not be in possession of a rail card .
generally when selling on board from a station with ticket buying facilities I will remind the passenger of the need to buy before they board but will sell the ticket they request and the risk of getting a penalty fare if they dont .
more blockades at unmanned stations in my view would provide better deterrent than having staff on board the train who dont have the time to really do anything other than sell a ticket try and argue about a penalty with passengers.